Apparatus for the manufacture of magnesium oxide



vpril'l?, 1945. F. ELKINGTON ETAL APPARATUS FOR THE MANUFACTURE OF MAGNESIUM OXIDE original Filed Feb. 28, 1940 /IIIII main YllAl/V/IIIAIIV 4 l. /IVAYAVlVAIIIIl A. l) A I Il /IrEVArlr ll lllllll l.

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Patented Apr. 17, 1945 APPARATUS FOR THE MANUFACTUBE F MAGNESIUM OXIDE Frank Elkington, Sheield, England, and Heinz Henry Chesny, West Los Angeles, Calif.

original application February 2s,

321,274. Divided and this application 1940, Serial No. April 4,

1942, Serial No. 437,664. In Great. Britain May and slaked dolomite and a dilute brine such as sea Water in order to form magnesium hydroxide and various proposals have been made for controlling the conditions of the reaction so that the magvnesium hydroxide is obtained in the desired physical form. The magnesium-hydroxide may be converted into magnesium oxide by a process of calcination and may be converted into other magnesium compounds. In particular it has been proposed to wash a slurry of magnesium hydroxide and to subject the washed slurry to spray drying in order to produce a dry powder of magnesium hydroxide without interfering with the amorphous nature of the precipitate. By introducing the nely divided magnesium hydroxide into an externally fired rotary retort and calcining it for a period of six to ten minutes at a temperature of 400 C., a product containing 70.84% MgO is obtained, the remainder of the product consisting of 14.38% magnesium hydroxide, carbonates, sulphates and impurities. nesium oxide thus obtained possesses physical activity in that it is capable of absorbing coloring matter with great readiness.

The present invention provides an apparatus which is highly desirable in the economical production of reactive magnesium oxide by the interaction of an aqueous slurry of calcined and slaked dolomite and diluted aqueous brine such as sea water. The resulting product is particularly suitable for the manufactureof metallic magnesium and for the production of magnesia cement.

As employed herein and applied to magnesium oxide, the term reactive is` intended. to denote a magnesium oxide which possesses the property of setting rapidly when mixed with magnesium chloride solution to form a magnesia cement. The degree of reactivity is dependent upon the conditions prevailing during the passage of the material through the kiln and upon the extent to which the magnesium hydroxide is converted into magl nesium oxide.

The magvto provide a 2 claims. (ci. 23-219) for the production of reactive magnesium oxide is disclosed fully. That process comprises treatting an aqueous slurry of calcined and slaked dolomite with a dilute aqueous brine suchas sea water to form a crystal magnesium hydroxide, removing the magnesium hydroxide from the liquid and introducing it while in the wet state into an internally tired rotary kiln and calcining it at a temperature of the order of 850 C. to 1000l C. for a periodof the order of one to two hours. We have found that in order to produce a reactive material, it is highly essential that the calcination be carried out while the material is substantially out of contact with the flame itself. Accordingly, the principal object of the present lnvention is to provide a suitable combination rotary kiln and combustion furnace in which coal, oilA or gas is burned under conditions giving a very high temperature such as 1400 C. and over and in which the calcination can be carried out without bringing the material into contact with the ilame.

We have also found that the gaseous mixture must be relatively low in residual oxygen and consequently it is one of the objects of this invention kiln which is of such character that proper combustion cank be obtained without any appreciable quantity of residual oxygen being brought into contact with the material. We have found it' desirable to use a kiln in which the hot gases can be introduced therein at a temperature of 850" C. to -`l000 C. and will, during their passage through the kiln, coolA appreciably so that when discharged from the exit end, they will have a temperature inthe neighborhood of 200 C.

The crystalline magnesium hydroxide is introduced in the form of a slurrv containing about 20% to-25% MffOH'f into the upper end of the inclined kiln. AThe initial drying action causes the formation of a very stift', gum-like mass and it is essential to provide means of breaking up this mass into small and uniformly sized pieces.

'For'tbis purr-ose cutting blades, attached to the sides of the kiln and disposed at an angle of about 16". from tbe'vert'cal are provided. The length nf theportlnn of the kiln tted with these cutting blades is so arranged that all the material is cut into masses from one inch to two inches in r-ian1eter. Subsequently, the material is passed into a zone in the kiln in which lifting troughs disposed slightly inclined in relation to the axis of the kiln are provided. These lifters serve the pur-rose of picking up the partially calcined material and dropping it through the stream of hot gases. It is necessary to provide mechanical the eiiect of breaking up the means for bringing the material into contact with the gas stream in order to obtain a uniform and reactive material.

The length, the angle ci' inclination and the speed of rotation of the kiln, are so selected that the material passes through the kiln within a period of from one to two hours. The last section at the lower end of the kiln is free from any mechanical means for distributing the material. The rate of movement of the material along the kiln is so governed that the material is moved rapidly towards the zone of maximum temperature.

In a modification of the invention, the magnesium hydroxide slurry is dried in a suitable dryer, such as a tray dryer, which affords a relatively slow drying action. Thus, small lumps of magnesium hydroxide, which may still contain a substantial proportion of free water are formed. Subsequently, the dried lumps are introduced into a rotary kiln, such as described above.

The most reactive material contains, on an impurity-free basis, from 10% to 50% MgO and from 90% to 50% Mg(OH)2. Thus, one example of a product made in accordance with the present invention had the following analysis:

MgO 28.0 Mg(OH 2 69.4 CaO 1.2 sioz 0.7 A1203 0.3 F8203 0.4

This is equivalent to '76.0% of MgO.

When mixed with magnesium chloride solution (S. G. 1.25) in the proportion of 100 grams of magnesium oxide equivalent to 125 cubic centimeters of magnesium chloride solution, this material gives an initial set on a, standard Vicat apparatus in l to 15 minutes and sets up as a hard cement within 50 to 80 minutes. It has been found that such a product is eminently suited to the production of metallic magnesium.

In the accompanying drawing we have shown f or purposes of illustration only, a preferred embodiment and one modified form of kiln embodying our invention. We have also shown diagrammatically a process in which our apparatus is particularly useful. In the drawing,

Figure l is a flow sheet of the process whereby dolomite is calcined by being passed through a kiln, the calcined material slaked with water and then converted to a slurry which is reacted with sea water to form a crystalline magnesium hydroxide and the magnesium hydroxide converted to magnesium oxide by passage through an internally red rotary kiln embodying our invention;

Figure 2 is a section through a. preferred embodiment of our invention; and

Figure 3 is a sectional view of a modified form of rotary kiln provided by our invention.

Referring to Figures 2 and 3, the inclined rotary kiln consists of a shell II which is lined with refractory brickwork I2. The kiln is supported on bearing rings I3 running on rollers Il and is provided with cutting blades I5 attached to the wall of the kiln and inclined at an angle of e. g. 16 to the vertical. -These blades have partially dried magnesium hydroxide slurry. After passing the zone of the kiln provided with cutting blades the material passes a series of lifting blades Ii adapted to lift the material and to drop it through the body of' hot gases passing through the kiln. The lifting blades are only slightly inclined to the horizontal and are preferably slightly curved so as to retain the material until it reaches a point near the top of the kiln before being dropped. The upper and lower ends of the kiln are preferably free from mechanical means for cutting or lifting the material.

Referring to Figure 2, the lower end of the kiln4 is dared at II in order that the calcined material may pass quickly through the hottest zone of the kiln to radially disposed holes or slots I8 provided with nap valves ISI lined with refractory material and arranged to open automatically (as at 20) when at the underside of the kiln. The remainder of the kiln is tapered at 2| towards the burner 22. A hood 23 is arranged over the central portion -of the burning zone containing the valves I9 so as to provide a reasonably tight seal to prevent inward leakage of air or outward leakage of combustion gases. The hood 23 is also provided with an opening at the bottom (not shown) for the discharge of the calcined material.

Referring to Figure 3, the lower end of the rotary kiln is connected to one end of a stationary combustion furnace 24. An oil, gas or pulverlzed coal burner 25 is provided at the opposite end of the combustion furnace .24. The furnace has a firing wall 26 for the purpose of preventing the flame from the furnace from passing into the rotary kiln itself. A lcircular hole 21 of a diameter approximately equal to the inside diameter of the kiln is arranged in the firing wall 26 and the hot combustion gases pass from the furnace through this hole and enter the kiln.

Suitable brick receiving chambers may 'be provided at the feed end of the kiln and may be connected either to' a chimney or to a fan creating the necessary suction.

A slurry of crystalline magnesium hydroxide obtained by the interaction of sea water and a slurry of calcined and slaked dolomite and containing, for example, per litre of slurry:

Grammes Magnesium hydroxide 300 Calcium sulphate 0.5 Calcium chloride 0.7 Calcium silicate 3.0 Iron oxide 1.5 Aluminum oxide 1.0 Calciumcarbonate 0.07 Sodium :'hlm-ifln 0,3

is fed at an even rate into a slowly revolving rotary kiln of the type shown in Figure 2 or Figure 3.

In the furnace shown in Figure 3 heat is obtained by the combustion of coal, oil or gas in a stationary combustion furnace 24,l the temperature of the latter being maintained at 1250 to 1450" C. The hot combustion gases from the furnace 24 enter the rotary kiln Il at a temperature of 850 C. Draught is applied tothe rotary kiln by means of a. stack or a fan for the purpose of drawing the gaseous mixture through the kiln at a velocity suiiiciently high to heat the slurry and the solids resulting therefrom but not sumciently high to pick up solid material in the gas stream. Depending upon the length4 of the kiln, the temperature the temperature at the feed end of the kiln, i. e. the gas exit end, lies in the range -oi' 150 to 250 C.

Alternatively, we may employ a rotary kiln having a iiared iiring section lined with refractory brickwork and re directly into the kiln as conditions are adjusted so that' shown in Figure 2. It is the purpose of the dared end section I1 to support the combustion or the fuel, viz. powdered coal or oil, by the heat retained by and radiated in this section. As it is of great importance that the calcined material should not be subjected to direct flame, the flared section I1 permits very rapid passage of the ma terial through the hot zone. Thus, we are able to calcine the magnesium hydroxide at a relatively low temperature while maintaining the flared section l1 at a temperature suniciently high to ignite and properly burn the fuel.

The rate of feeding and the rate of rotation of the kiln are adjusted so that the material passes through the entire length of the kiln within a period of time ranging from one to two hours.

We claim:

1. Apparatus for the production of magnesium oxide from magnesium hydroxide comprising an internally-tired inclined rotary kiln the lower and ring end of which is nared and subsequently tapered to the burner, the portion joining the flared and tapered sections of the kiln being apertured rfor the removal of calcined material from the kiln, each aperture being provided with a refractory-lined flap valve which opensautomatically when the aperture with which it is associated is at the underside of the kiln.

2. Apparatus as claimed in claim 1 wherein the apertured portion of the kiln is provided with a substantially gas-tight hood preventing leakage of air into or combustion gases out of the kiln.

FRANK ELKINGTON. HEINZ HENRY CHESNY. 

